I've never cut them off for him, nor has he witnessed anybody else doing so. He sometimes eats them and sometimes not. Same as me!
I would probably be reluctant to give him another sandwich if he left a lot of the first one on his plate. Luckily, it has never occurred to him to toss the crusts aside and ask for another helping. I've seen other kids take a couple soft bites from the middle and consider the rest of the sandwich to be garbage. I'm not sure how I'd deal with that. Probably not by making another sandwich.
But generally we're not big on seconds anyhow. I'd rather have him choose some fruit or cheese than have a second PB&J, for variety. My DS is not difficult about food though, so there's no struggle there.